1. Field of the Invention
The present invention relates to a solid state image sensor, and more particularly, to such a sensor having a reading mechanism of the charge transfer device type such as a charge coupled device (called "CCD" in this specification).
2. Description of Related Art
At present, a solid state image sensor using a charge coupled device (called "CCD image sensor" hereinafter) is being reduced into practice as an imaging means for a video camera. Referring to FIG. 1, there is shown a layout pattern of a typical conventional CCD image sensor. The shown CCD image sensor comprises a semiconductor substrate 10 of a given conduction type, on which a number of photo-electric converting elements (not shown) are arranged in the form of a matrix having a plurality of rows and a plurality of columns. Each column of photoelectric converting elements is associated with a vertical transfer register composed of a CCD device so that electric signals outputted from the respective photo-electric converting elements are transferred in parallel to the associated vertical transfer CCD device. In FIG. 1, for simplification of the drawing, only a portion of one vertical transfer register is shown. In the shown example, the vertical transfer CCD device includes a plurality of first vertical transfer electrodes 12 and a plurality of second vertical transfer electrodes 14, which are formed through an insulating layer on the semiconductor substrate 10, and alternately arranged in a regularly repeated pattern in the vertical direction along the associated photo-electric converting element column. The first vertical transfer electrodes 12 and the second vertical transfer electrodes 14 partially overlap with one another in the plan view, but are electrically isolated from one another. In the case of a two-phase drive, a first pair of the first vertical transfer electrode 12 and the second vertical transfer electrode 14 adjacent to each other are applied with a first vertical transfer clock, and a second pair of the first vertical transfer electrodes 12 and the second vertical transfer electrodes 14, which are adjacent to each other and are positioned at each side of the first pair of vertical transfer electrodes, are applied with a second vertical transfer clock opposite in phase to the first vertical transfer clock, so that an electric charge is transferred along an array of the first vertical transfer electrodes 12 and the second vertical transfer electrodes 14 with application of the first and second vertical transfer clocks.
A termination of each vertical transfer CCD device is coupled to a corresponding stage of a horizontal transfer register, which is in turn composed of a CCD device. In FIG. 1, only a portion of one horizontal transfer register is shown also for simplication of the drawing. In the shown example, the horizontal transfer CCD device includes a plurality of first horizontal transfer electrodes 16 and a plurality of second horizontal transfer electrodes 18 which are formed through an insulating layer on the semiconductor substrate 10, and alternately arranged in a regularly repeated pattern in the horizontal direction perpendicular to the vertical transfer CCD device. The first horizontal transfer electrodes 16 and the second horizontal transfer electrodes 18 partially overlap with one another in the plan view, but are electrically isolated from one another. In the case of a two-phase drive, a first pair of the first horizontal transfer electrode 16 and the second horizontal transfer electrode 18 adjacent to each other are applied with a first horizontal transfer clock, and a second pair of the first horizontal transfer electrodes 16 and the second horizontal transfer electrodes 18, which are adjacent to each other and are positioned at each side of each first pair of horizontal transfer electrodes, are applied with a second horizontal transfer clock opposite in phase to the first horizontal transfer clock, so that an electric charge is transferred along an array of the first horizontal transfer electrodes 16 and the second horizontal transfer electrodes 18 with application of the first and second horizontal transfer clocks.
At an end of the horizontal transfer CCD device, there is formed an electric charge detection circuit, which includes a first output gate electrode 20 partially overlapping with the final horizontal transfer electrode of the horizontal transfer CCD device but electrically isolated from the final horizontal transfer electrode. A second output gate electrode 22 is further provided to partially overlap with the first output gate electrode 20 but to be electrically isolated from the first output gate electrode 20. In addition, an output transistor is formed adjacent to the second output gate electrode 22. This output transistor includes a first diffusion region 24 and a second diffusion region 26 formed in the substrate 10 separately from each other, and a reset or control electrode 28 formed between the first and second diffusion regions 24 and 26. With above arrangement, photo signals generated by the respective photo-electric converting elements arranged in the matrix are serially outputted from the output transistor.
On the other hand, in order to ensure that the CCD image sensor has a sufficient performance as an imaging means for a video camera, the CCD image sensor has to be composed of 300,000 or more picture elements, namely photo-electric converting elements. However, it is difficult to fabricate the 300,000 photo-electric converting elements perfectly identical to one another, because of dispersion in diffusion process. The larger the number of the photo-electric converting elements becomes, the greater the dispersion of the photo-electric converting elements also becomes. The dispersion of the photo-electric converting elements means that there is a dispersion in a saturation level of the photo-electric converting elements means. However, when the CCD image sensor is used in a video camera, the photo-electric converting elements will be used to be often brought into a saturation region or status. Therefore, if a sufficient amount of light to cause a group of photo-electric converting elements to saturate is inputted into the video camera, unevenness will appear in a saturated image portion because of the dispersion in the saturation level of the photo-electric converting elements. As a result, an image quality will be deteriorated. However, the conventional CCD image sensors used in the video camera have no means for restraining the dispersion in the saturation level of the photo-electric converting elements. Therefore, the CCD image sensor having the dispersion in the saturation level of the photo-electric converting elements must have been rejected as a defective. In other words, the production yield of the conventional CCD image sensors has been low.
As seen from the above, the conventional CCD image sensors have often deteriorate the image quality or have been low in yield of production.